1. Field of the Invention
The present invention relates to a method for uniformly distributing the vapors leaving the parallel flow condensing section of an air cooled condenser into the dephlegmator tubes of a subsequently arranged countercurrent condensing section. The present invention also relates to an air cooled condenser for carrying out the method.
2. Description of the Related Prior Art
The use of air for the condensation of turbine vapor has long been known in the art. In the case of direct air condensation, the turbine vapor is condensed in fin tube elements arranged in parallel followed by the condensate being returned to the feed water cycle. The interior of the fin tube elements is under negative pressure, and entrained gases which cannot be condensed are withdrawn. The cooling air flow is generally produced by means of fans or, less frequently, by natural draft.
The air cooled condensers are arranged in modular fashion with the fin tube elements arranged vertically, horizontally or in an inclined A-like or V-like configuration,
The roof-like or A-like configuration is widely used. In that case, the fin tube elements form two legs of a triangle and the fans form the third leg at the base of the triangle.
A major problem in air cooled condensers is the fact that freezing of the fin tube elements must be prevented during the winter months, particularly during operation with partial load, and the danger of freezing must be reliably eliminated with the use of apparatus which is as simple as possible and inexpensive.
Two configurations of air cooled condensers are common. They are the parallel flow condenser configuration and the countercurrent condenser configuration, also referred to as a dephlegmator.
In the case of the parallel flow condenser, the vapor flows downwardly in fin tubes fed from an upper distribution duct. The pressure drop in the fin tubes causes a temperature drop of the saturated vapor.
This temperature reduction essentially results in a drop of the operative temperature difference between the vapor and the cooling air, so that the efficiency of heat removal of the condenser is reduced.
Another more serious consequence is the fact that vapor in the fin tubes is completely condensed before it reaches the tube ends. This may occur in the case of low air temperatures or when operating under partial load. In that case, the condensate subcools very quickly and gases which cannot be condensed collect in the remaining tube sections in which no vapor is present. This leads to an increase of the oxygen absorption of the condensate which, in turn, may lead to corrosion problems. Moreover, subcooling of the condensate may result in freezing of the condensate when the air temperature is below 0.degree. C., so that there is the danger that the cooling tubes are damaged or that they burst.
DE-AS 10 44 125 already discloses a proposal which has the purpose of preventing the freezing of the condensate in parallel air cooled condensers. In that case, the heat exchanger surfaces of the fin tubes are adjusted to the available temperature drop between the vapor entry temperature and the cooling air temperature in such a way that the condensation is concluded as uniformly as possible in all tube rows at a small distance from the ends of the tube rows where they connect to the condensate collection manifold. In order to achieve this vapor distribution, devices for throttling the vapor intake in the form of nozzles or shields are provided at the inlet side of the fin tubes.
It is a disadvantage that these shields are arranged at the inlet side because the entire vapor to be condensed in the condenser must flow through these shields and is distributed with a pressure loss into the fin tubes which are arranged one behind the other on the air side.
In addition, the disadvantages resulting from subcooling of the condensate can be prevented by using the above-mentioned countercurrent condenser configuration.
In that type of operation, the vapor is introduced from below into the fin tubes and is conducted in a countercurrent flow against the condensate which is draining off. Since the vapor continuously transfers heat to the condensate draining in the opposite direction, there is the advantage that subcooling of the condensate cannot occur when the apparatus is correctly dimensioned.
The countercurrent condenser configuration has the disadvantage of operating at a reduced heat transfer coefficient. Moreover, the possible condensation rate of a countercurrent condenser can be reduced if slug flow conditions exist which produce a holdup of the condensate in the fin tubes. Slug flow conditions are that state of operation in which the vapor introduced from below into the dephlegmator tubes and flowing upwardly can no longer flow against the mass of the downwardly flowing condensate. This causes a condensation holdup in the fin tubes.
A solution which has proved successful in practice is the combination of a parallel flow condenser and a countercurrent condenser, as disclosed, for example, in DE-PS 11 88 629.
In that case, fin tube elements operating as dephlegmators are arranged downstream of the fin tube elements operating as parallel flow condensers. The fin tube elements operating as dephlegmators are simultaneously arranged in groups in cooling sectors in such a way that, when operating under partial load and at external air temperatures below the freezing point during the winter months, at least a portion of the element groups operating as parallel flow condensers can be switched off on the vapor side as well as on the air side, so that the vapor is condensed predominantly in the element groups operating as dephlegmators. While the countercurrent condensers have a poorer efficiency as compared to the parallel flow condensers, they have the advantage that they do not freeze even when operating under partial load because of the continuous contact of the downwardly flowing condensate with the upwardly flowing vapor.
The so called condensation end of the vapor is then located in the countercurrent condenser, so that subcooling of the condensate is generally avoided. The system is regulated by switching off individual cooling sectors or by changing the cooling air flow.
In order to achieve a uniform distribution of the vapor flow introduced into the vapor distribution chamber of a countercurrent condenser with a relatively low flow speed, it is additionally known from DE-GM 18 73 644 to provide an intermediate sheet with openings in the vapor distribution chamber. The entire cross sectional area of the openings is smaller than the total cross sectional area of the condenser tubes.
This solution also has the purpose of regulating and uniformly distributing the vapor entering the individual condenser tubes.
The combined arrangement of fin tube elements as condensers and dephlegmators has proved successful in operation. However, in order to be able to handle very large load variations, particularly low vapor quantities during the winter months, without the danger of subcooling or freezing, it is also necessary in this arrangement to provide additional means for regulating and controlling the flow and quantity of the cooling air and the exhaust vapor.
It must be taken into consideration in this connection that, due to the parallel arrangement of all fin tube elements on the vapor side as well as on the condensate side, and in the case of admitting varying quantities of cooling air to the various groups, the pressure loss of the vapor flow is equal in all fin tube elements. This has the consequence that more vapor flows through the less strongly cooled group than could be condensed in this group, while simultaneously less vapor flows through the more strongly cooled groups than could be condensed in those groups. While the effect mentioned first has the disadvantage that excess vapor is withdrawn through the exhaust line to the vacuum system and, thus, the vacuum is negatively affected, the effect mentioned last has the disadvantage that the vapor is not admitted fully over the entire length of the fin tube elements and, consequently, there is still the danger of freezing in the case of very low ambient air temperatures.
In addition, particularly in the case of high loads, a condensate holdup may occur in the cooling tubes of the counterparallel flow condenser, with the result that vapor penetrates into the upper non-condensible gas collector and then enters the fin tubes from above producing so called cold pockets in the fin tubes in which inert gases collect, so that the efficiency of the countercurrent condenser is reduced.